Crystal structure



7.. 8 a.. I? A.E 3 3 Aug. 15, 1967 L.. G. MARTYN ETAL CRYSTAL STRUCTUREFiled April 1s, 1965 FIG. 2

FIG. 1

FIG. 3

ny wm r an M. GB Wm lo@ LA y B WMO United States Patent O Park, lll., acorporation of illinois Filed Apr. 13, 1965, Ser. No. 447,626 9 Claims.(Si. S10- 9.5)

This invention relates generally to piezo-electric crystal devices, andmore particularly to such a crystal device operating at a high orderovertone for use in high frequency filter circuits.

Piezo-electric crystals have been used in filter circuits for manyyears. Such crystals have been used mainly in circuits operating at lowfrequencies below one megacycle. Problems have been encountered inproviding crystal filters for operating at higher frequencies in therange from 7 megacycles to much higher frequencies, such as frequenciesin the range from 135 to 175 megacycles, for example. For operation atthese higher frequencies, AT-cut crystals may be used at odd orderovertones such as the third, fifth and seventh overtones. In suchoperation, spurious responses have been produced by unwanted modes ofoperation of the ciystals. These spurious responses may occur atfrequencies relatively close to the frequency of operation, and therebyprovide undesired response characteristics in the crystal filtercircuits in which they are used.

lt is, therefore, an object of the present invention to provide animproved crystal device for use in crystal filter circuits operating inthe frequency range above 7 megacycles.

Another object of the invention is to provide a piezoelectric crystaldevice for operating at a high order overtone, such as the fifth orseventh, and in which spurious responses at frequencies adjacent theoperating frequencies are of low amplitude.

Another object of the invention is to provide a crystal for overtoneoperation having a low series resistance at the frequency used and veryhigh resistance at frequencies in the vicinity of the overtonefrequency, at which unwanted modes of operation may appear.

A further object of the invention is to provide an overtone crystal foruse in filter circuits wherein nonlinear diode effects are minimized.

A feature of this invention is the provision of an overtone crystaldevice including a fiat circular A'l`cut crystal blank with circularelectrodes on the opposite fiat faces thereof, and wherein the diameterof the crystal blank is no more than 8 times the diameter of theelectrode. A plurality of electrodes may be placed on each dat face ofthe blank and in such case the diameter of the blank should be no morethan 8 times the sum of the diameters of the electrodes on one side ofthe blank.

Another feature of the invention is the provision of a crystal deviceincluding a fiat AT-cut crystal blank with electrodes on opposite faces,and a conducting shielding cover with conductors extending therefromwhich are connected to the electrodes, and wherein the static capacitybetween the conductors is in the range from 0.9 to 1.1.micromicrofarads. The capacity between the conductors includes thecapacity resulting from the size of the electrodes and the thickness ofthe blank, and in addition the capacity resulting from the action of theconducting shielding cover.

A further feature of the invention is the provision of a crystal devicefor operation at its seventh overtone in the frequency range from 135 to175 megacycles, wherein the diameter of the blank lies in the range from.l to .2 inch and the electrode diameter does not exceed .025 inch. Theelectrodes are formed on the crystal by a single Fice plating so thatthere is no junction between dis-similar materials to provide anonlinear diode effect.

The invention is illustrated in the drawing wherein:

FIG. 1 illustrates a crystal device in accordance with the inventionhaving a single pair of electrodes;

FG. Z is a side view of the crystal device of FIG. 1; and

FIG. 3 illustrates a crystal device having a plurality of pairs ofelectrodes.

1n practicing the invention there is provided a piezoelectric crystaldevice for operating at an overtone and to be used in a high frequencycrystal filter circuit. The crystal blank is AT-cut so that thethickness determines the frequency of operation. The flat crystal blankis preferably circular in configuration with circular electrodesthereon. One or more pairs of electrodes may be applied, with each pairhaving electrodes positioned opposite each other on the flat sides ofthe blank. Electrodes are provided by plating a single material so thereis no junction of dis-similar materials which will provide nonlineardiode effects. The crystal blank has a diameter in the range from 4 to 8times the diameter of the electrode when a single electrode is providedon each side of the blank, and if a plurality of electrodes are providedthe f diameter of the blank should be in the range from 4 to 8 times thesum of the diameters of the electrodes on each side. The static capacityof the crystal device as evidenced at its terminals should lbe in therange from 0.9 to l.l micromicrofarads. This includes the capacitycontrolled by the diameter of the electrodes and the thickness of thecrystal blank, and the additional capacity produced by the crystalenclosure. The thickness of the planting of the crystal is kept low, andin the construction of a crystal for operation in the range from 135 to175 megacycles, for example, the thickness of the blank without platingshould be at least percent of the total thickness of the blank with theelectrodes plated on the two sides thereof.

In the drawing, FIGS. 1 and 2 illustrate a crystal device in accordancewith the invention. The crystal blank 16 is a thin plate ofpiezo-electric material having a circular periphery, and may be anAT-cut from a quartz crystal. A'n electrode 12 of a material such asaluminum is plated on each side of the crystal. The electrodes properare of circular configuration and the electrodes on opposite sides areentirely coextensive. Extensions 13 on the platings form connections tothe edge of the crystal blank 10. The crystal is supported on a base 15through which conducting pins 16 extend for making electricalconnections to the crystal electrodes, and also to provide a mechanicalplug-in connection. Resilient wires 17 secured to the conducting pins 16have clip portions 18 which extend on both sides of the crystal blank 16to support the same. The clip portions engage the extensions i3 of theplatings so that electrical connections are lmade to the two electrodes12 on the opposite sides of the blank 10. A cover 2t) is secured to themounting base and forms an enclosure for the crystal. This is made of aconducting material to form a shield.

The crsytal device is constructed to operate in an overtone made in ahigh frequency filter circuit. Unwanted modes of vibration producingspurious responses in the vicinity of the overtone are reduced by use ofa crystal blank having a diameter no greater than 8 times the diameterof the electrodes 12, and preferably in the range from 4 to 8 times theelectrode diameter. For operating at .the seventh overtone in thefrequency range from to 175 rnegacycles, the diameter of the crystalblank may be in the range from .l to .2 inch, with the electrodediameter not exceeding .025 inch. In such a crystal the seriesresistance at the seventh overtone is relatively low, being in the rangebetween 200 and 600 ohms, and the resistance assets? t for other modesof vibration in the vicinity of the seventh overtone is very high andmay be greater than 100,060 ohms. This minimizes the spurious responsesclosely adjacent to the operating frequency.

It has been found that the static capacitance between the terminal 16 ofthe crystal device should be of the order of 0.9 to 1.1micromicrofarads. This capacitance is made up of two main factors, thecapacitance resulting from the electrodes with the crystal therebetweenand the capacitance resulting from the coupling to the cover. Thecapacitance between the electrodes may be defined as DeZ/T, where De isthe diameter of the electrodes i2, and T is the thickness of the crystalblank llii. The static capacitance also includes the capacitanceresulting from the coupling of the conducting cover to the electrodesand conductors extending therefrom, The two elements of capacity may beon the same order of magnitude.

Another factor which may be of importance in minimizing the spuriousresponses is to hold the thickness of the electrodes to a minimum valuewhich provi-les reliable plated electrodes on the crystal blank. Forcrystals operating in the frequency range from 135 to 175 megacycles,the thickness of the plating should be such that the thickness T of thecrystal blank alone is at least 95 percent of the total thickness TP ofthe blank and platings.

In FIG. 3 there is illustrated another embodiment of the invention, andin this construction the crystal may operate as two separate sectionswith the sections having the same, or somewhat different frequencyresponses. The crystal blank 2.5 has thereon a iirsL set of electrodeswhich are of circular configuration and coextensive on the two sides ofthe crystal blank, and a second set of electrodes 27 which are similarlycoextensive on t'ne two sides of the blank. The electrodes are plated onthe blank, and the platings extend from the electrode 26 on one side toform a connector 28 thereto, and from the electrode on the other side ofthe crystal blank to form connector 29. Similarly connectors 30 and 31extend from the electrodes 27. The connector extensions 28 and 30 may beengaged by clips of supporting wires 32 and 33 as in the structure ofFIG. 1. The connectors 29 and 31 join together and are connected to athird supporting wire 35 which may also be of the same construction asin FIG. 1. lf desired, the connectors 29 and 31 can be separate andengaged by separate clips, but in many applications one electrode ofeach section may be held at a common potential.

In the structure of FIG. 3 the same basic considerations apply as setforth in connection with the structure of FiG. 1. By control of thethickness of the plating of the electrodes, the two crystal sections mayhave series resonant frequencies spaced from each other. The sectionscan then be used in a bandpass crystal lter circuit. The two sets ofelectrodes are mounted on the crystal blank at approximately withrespect to the XX or electrical axis of the quartz blank, and also 45with respect to the ZZ or optical axis of the blank. This reduces thecoupling between the sections. In order to reduce undesired spuriousresponses, the ratio of the diameters of the blank and the electrodemust take into account the two electrodes on each side of the blank.'Ihe diameter of the blank should be 4 to 8 times twice the diameter ofthe individual electrodes, or 4 to 8 times the sum of the diameters ofthe two electrodes on each side. For operating in the frequency rangebetween 135 and 175 megacycles, the diameter of the blank may be .400inch and the diameter of each electrode may be .025 inch. The capacitybetween the contact pins connected to each pair of electrodes should bewithin the range of 0.9 to 1.1 micromicrofarads.

It may be desired in certain instances to provide more than two pairs ofelectrodes on a crystal blank. It would be possible to provide 4 pairsof electrodes, with the third and fourth pairs positioned alonga line atright angles to the line through the tirst and second pairs as shown inFIG. 3. 'I'hen each pair would be at 45 to both the electrical andoptical axis of the crystal blank. In such case the diameter of theblank should be from 4 to 8 times the sum of the diameters of the fourelectrodes on each side of the blank or 4 to 8 times 4 times thediameter of each electrode. The plating extensions forming connectionsto the electrodes should be arranged so that they are not directlyopposite each other on the two sides of the blank, and can be entirelyseparate or joined in pairs as may be desired for particular circuitapplications. The crystal device would therefore act as four separatecrystal sections and could be used in a four crystal filter circuit.

The additional plating on the crystal blank providing a plurality ofelectrodes, and for the connectors therefor, acts to stilien the crystalblank. This increases the power handling capability of the blank. It maybe desired to apply platings which are not connected in a circuit toprovide added stiffness to the crystal blank.

Crystals constructed as has been described have operated at highfrequencies and have been substantially free of spurious response. Suchcrystal have operated satisfactorily in the frequency range between 135and 175 megacycles. Filter circuits using a plurality of crystalsections on a single blank, as described, have provided the desiredcharacteristics.

l/Ve claim:

l. A crystal device including in combination, a thin flat AT-cut crystalblank having a circular periphery, first and second electrodes ofcircular configuration secured to opposite flat sides of said blank, amounting base, support means for said blank including iirst and secondconductors connected respectively to said first and second electrodes,said conductors being supported by said mounting base and havingportions extending therethrough for connection to a circuit, andconducting cover means supported by said mounting base and cooperatingtherewith to enclose said crystal blank and said electrodes, saidcrystal blank having a diameter in the range from 4 to S times thediameter of each electrode, and the static capacity between saidconductors including the capacity between said electrodes with the blanktherebetween, and the capacity resulting from said cover, being in therange from 0,9 to 1.1 micromicrofarads.

2. A crystal device -for operating in the frequency range from 135 to175 megacycles including in combination, a thin fiat AT-cut crystalblank having a circular periphery, first and second electrodes ofcircular contiguration secured to opposite flat sides of said blank,said electrodes being formed by conductive platings on said blank whichhave portions extending from said electrodes, a mounting base, supportmeans for said blank including iirst and second conductors supported bysaid mounting base and having portions extending therethrough forconnection to a circuit, resilient conducting wires connected to saidfirst and second conductors and supporting said blank and engaging saidportions extending from said rst and second electrodes respectively andconducting cover means supported by said mounting base and cooperatingtherewith to enclose said crystal blank and said electrodes, saidcrystal blank having a diameter in the range from 4 to 8 times thediameter of each electrode, and the thickness of said blank alone beingat least percent of the total thickness of said blank and said platingsthereon.

3. A crystal device including in combination, a thin dat AT-cut crystalblank having a circular periphery, Iirst and second electrodes ofcircular configuration secured to opposite dat sides of said blank, saidelectrodes being formed by conductive platings on said blank which haveportions extending from said electrodes, a mounting base, support meansfor said blank including first and second conductors supported by saidmounting base and having portions extending therethrough for connectionto a circuit, resilient conducting wires connected to said rst andsecond conductors and supporting said blank and engaging respectivelysaid portions extending from said first and second electrodes, andconducting cover means supported by said mounting base and cooperatingtherewith to enclose said crystal blank and said electrodes, saidcrystal blank having a diameter in the range from 4 to 8 times thediameter of each electrode, the static capacity between said conductorsincluding the capacity between said electrodes with the blanktherebetween and the capacity resulting from said cover being in therange from 0.9 to 1.1 micromicrofarads, and the thickness of said blankalone being at least 95 percent of the total thickness of said blank andsaid platings thereon.

4. A crystal device for operating in the frequency range from 135 to 175megacycles including in combination, a iiat AT-cut crystal blank havinga circular periphery, first and second electrodes of circularconfiguration secured to opposite flat sides of said blank, a mountingbase, support means for said blank including iirst and second conductorsconnected respectively to said first and second electrodes, saidconductors being supported by said mounting base and having portionsextending therethrough for connection to a circuit, and conducting covermeans supported by said mounting base and cooperating therewith toenclose said crystal blank and said electrodes, said crystal blankhaving a diameter in the range from 0.1 to 0.2 inch and the diameter ofeach electrode being no greater than 0.25 inch.

5. A crystal device for operating in the frequency range from 135 to 175megacycles including in combination, a fiat AT-cut crystal blank havinga circular periphery, rst and second electrodes of circularconfiguration secured to opposite fiat sides of said blank, a mountingbase, support means for said blank including iirst and second conductorsconnected respectively to said first and second electrodes, saidconductors being supported by said mounting base and having portionsextending therethrough for connection to a circuit, and conducting covermeans supported by said mounting base and cooperating therewith toenclose said crystal blank and said electrodes, said crystal blankhaving a diameter in the range from 0.1 to 0.2 inch and the diameter ofeach electrode being no greater than .025 inch, the static capacitybetween said conductors including the capacity between said electrodeswith the blank therebetween and the capacity resulting from said coverbeing in the range from 0.9 to 1.1 micromicrofarads.

6. A crystal device including in combination, a iiat AT-cut crystalblank having a circular periphery, a plurality of electrodes of circularconfiguration positioned in pairs on opposite sides of said blank, amounting base, support means for said blank including a plurality ofconductors connected to said electrodes and supported by said mountingbase, said conductors having portions extending through said mountingbase for connection to a circuit, and cover means supported by saidmounting base and cooperating therewith to enclose said crystal blank,said crystal blank having a diameter in the range from 4 to 8 times thesum of the diameters of the electrodes on one side of said blank, andthe capacity between said conductors connected to each pair ofelectrodes including the capacity Ibetween said electrodes with theblank therebetween and the capacity resulting from said cover being inthe range from 0.9 to 1.1 micromicrofarads.

7. A crystal device including in combination, a fiat AT-cut crystalblank having a circular periphery, a plurality of electrodes of circularconfiguration positioned in pairs on opposite sides of said blank, saidelectrodes being formed by conductive platings on said blank which haveportions extending from said electrodes, a mounting base, support meansfor said blank including a plurality of conductors engaging said platingextensions and mechanically connected to said blank and supported bysaid mounting base, said conductors having portions extending throughsaid mounting base for connection to a circuit, and cover meanssupported by said mounting base and cooperating therewith to enclosesaid crystal blank, said crystal blank having a diameter in the rangefrom 4 to 8 times the sum of the diameters of the electrodes on one sideof said blank, and the static capacity between said conductors connectedto each pair of electrodes including the capacity between saidelectrodes with the blank therebetween and the capacity resulting fromsaid cover being in the range from 0.9 to 1.1 micromicrofarads.

8. A crystal device for operating in the frequency range from 135 to 175megacycles including in combination, a fiat AT-cut crystal blank havinga circular periphery, first and second pairs of electrodes of circularcontiguration positioned each having an electrode on each iiat side ofsaid blank, said electrodes being formed by conductive platings on saidblank which have portions extending from said electrodes, a mountingbase, support means for said blank including a plurality of conductorsengaging said plating extensions and mechanically connected to saidblank and supported `by said mounting base, said conductors havingportions extending through said mounting base for connection to acircuit, and cover means supported by said mounting base and cooperatingtherewith to enclose said crystal blank, said crystal blank having adiameter in the range from .2 to .4 inch, and the diameter of eachelectrode being no greater than .025 inch, and the static capacitybetween said conductors connected to each pair of electrodes includingthe capacity between said electrodes with the blank therebetween and thecapacity resulting from said cover being in the range from 0.9 to 1:1micromicrofarads.

9. A crystal device for operating in the frequency range from 135 to 175megacycles including in combination, a iiat AT-cut crystal blank havinga circular periphery, a plurality of electrodes of circularconfiguration positioned in pairs on opposite sides of said blank, saidelectrodes being formed Iby conductive platings on said blank which haveportions extending from said electrodes, a mounting base, support meansfor said blank including a plurality of conductors engaging said platingextensions and mechanically connected to said blank and supported bysaid mounting base, said conductors having portions extending throughsaid mounting base for connection to a circuit, and cover meanssupported by said mounting base and cooperating therewith to enclosesaid crystal blank, said crystal blank having a diameter in the rangefrom 4 to 8 times the sum of the diameters of the electrodes on one sideof said blank, the static capacity between said conductors connected toeach pair of electrodes including the capacity between said electrodeswith the blank therebetween and the capacity resulting from said coverbeing in the range from 0.9 to 1.1 micromicrofarads, and the thicknessof said blank alone being at least percent of the total thickness ofsaid blanks and said platings thereon.

References Cited UNITED STATES PATENTS 2,343,059 2/ 1944 Hight S10-9.52,571,167 10/ 1951 Ruggles B10- 9.5 2,699,508 1/ 1955 Fastenau 31o-9.52,795,708 6/ 1957 Brooks 310--9.5 2,967,958 1/ 1961 Kosowsky S10-9.5

MILTON O. HIRSHFIELD, Primary Examiner.

I. D. MILLER, Assistant Examiner.

1. A CRYSTAL DEVICE INCLUDING IN COMBINATION, A THIN FLAT AT-CUT CRYSTALBLANK HAVING A CIRCULAR PERIPHERY FIRST AND SECOND ELECTRODES OFCIRCULAR CONFIGURATION SECURED TO OPPOSITE FLAT SIDES OF SAID BLANK, AMOUNTING BASE, SUPPORT MEANS FOR SAID BLANK INCLUDING FIRST AND SECONDCONDUCTORS CONNECTED RESPECTIVELY TO SAID FIRST AND SECOND ELECTRODES,SAID CONDUCTORS BEING SUPPORTED BY SAID MOUNTING BASE AND HAVINGPORTIONS EXTENDING THERETHROUGH FOR CONNECTING TO A CIRCUIT, ANDCONDUCTING COVER MEANS SUPPORTED BY SAID MOUNTING BASE AND COOPERATINGTHEREWITH TO ENCLOSE SAID CRYSTAL BLANK AND SAID ELECTRODES SAID CRYSTALBLANK HAVING A DIAMETER IN THE RANGE FROM 4 TO 8 TIMES THE DIAMETER OFEACH ELECTRODE, AND THE STATIC CAPACITY BETWEEN SAID CONDUCTORSINCLUDING THE CAPACITY BETWEEN SAID ELECTRODES WITH THE BLANKTHEREBETWEEN, AND THE CAPACITY RESULTING FROM SAID COVER, BEING IN THERANGE FROM 0.9 TO 1.1 MICROMICROFARADS.